I think you have it: most fights could not have involved the kind of lengthy blade play we see in most movies. It is really hard to correctly guess where your opponent will try to hit you, and equally as hard to effectively block that attack. Blocked attacks leave the opponent vulnerable to a (hopefully debilitating) counterattack, which means that the action could end very quickly, with a poorly conceived move possibly waiting for instant death. Armor protected the wearer, which allowed attackers to perhaps be more bold.

There's also the matter than in a great many sword fights, people crash into each other on accident. There's also any number of situations where punching your opponent in the face or grabbing his sword arm might be a lot easier that slapping blades around. Fighting manuals from the Late Medieval period include extensive depictions of hand-to hand grappling. This is something you almost never see in movie: our heroes, in armor, trying to pry his foe's helmet loose so he can jam a dagger into his face.

A book you could check out is Francis Geis, The Knight in History. It includes a telling of an encounter between William Marshall and a nearly equally skilled knight. After some maneuvering and striking about the armor, Marshall lunged forward and put his sword through the other mans' visor.

A fight in a dramatic presentation is acting, and is no more necessarily realistic than a rhymed soliloquy.

I own William Hobbes' book on fight arranging, and there are other good ones out there. Whether for stage or screen, they remind the student that fights need to be safe and serve a dramatic purpose, and contain drama. You have to show the characters gaining or losing confidence or determination in the course of the fight, and arrange it so that those changes seem natural to the audience. A classic example would be the final duel between Macbeth and MacDuff.

So movie fights expand to fill the time available, and use the riffs the fight arranger has come up with, and increase dramatic tension.

Your remark about thrusts, parries, and ripostes does remind me of how anachronistic a lot of movie medieval fighting has been. This began to change around 1972, when director Richard Lester handed Bill Hobbes authentic rapiers and told him to use them in the fights for The Three Musketeers.

The history of fight arranging with swords had always been a mixture of modern fencing and spectacle, starting with Elizabethan theatre. By the 19th C, the use of sbres and epees were way out of date. They also used to glue little flints to the blades so that they would throw off sparks in action. This was like the modern gunshots sparking when they hit steel grating: it's not right, but it's more exciting than reality!

If you watch The Adventures of Robin Hood